A portable electronic device usually has a power control circuit. This power control circuit is responsible for providing a supply voltage to a power amplifier that is configured to amplify and transmit signals, wherein this supply voltage is corresponding to a control signal which is sent from a baseband circuit of the portable electronic device. Based on different operation requirements of the portable electronic device, the control signal which is sent from the baseband circuit of the portable electronic device may sometimes be large and sometimes be small. However, the battery voltage of the portable electronic device will gradually decrease during the operation of the portable electronic device. If the control signal which is sent from the baseband circuit of the portable electronic device is large, the battery voltage of the portable electronic device may be insufficient to allow the power control circuit to provide a supply voltage which can correspond to the control signal. As a result, the power control circuit may not work normally.